Yes, I do drink diet coke, maybe a couple a day. I didn't know that it was so different, it is a possibility. I certainly prefer 'our' coke!
You're probably sensitive to the aspartame in the diet coke (it's banned here in the uk). You should try to avoid soda on your next trip and see if you feel better. The water you'd get in restaurants to drink and that is used for ice is filtered tap water. It really shouldn't be causing you any issues.